1. Field of the Invention
The present invention relates to an audio reproducing apparatus and a method thereof which are in particular suitable to be used for a digital power amplifier for reproducing and analog-outputting digital audio data recorded on a digital signal record medium such as a CD (Compact Disk).
2. Description of the Related Art
In the past, a PCM multi-bit method (hereafter, abbreviated as a PCM method) was adopted as means for representing audio information which is originally an analog signal as a digital signal. The PCM method is also adopted for a CD widely used today. In the case of the PCM method, an operation is performed according to quantization characteristics each time in timing of a sampling frequency (44.1 kHz) to replace the analog signal with the digital signal so as to record an absolute amount of data as to all sample points on the CD.
As opposed to this, attention is recently focused on a 1-bit method whereby distribution of quantization noise is controlled by using ΔΣ modulation and restorability from the digital signal to the original analog signal is thereby improved compared to the PCM method. In the case of the 1-bit method, only a variation against immediately preceding data is recorded as a binary signal without thinning out and interpolating an information amount as with the PCM method, and so a 1-bit signal obtained by quantization shows characteristics very close to those of the analog signal.
Therefore, an audio reproducing apparatus (digital power amplifier) based on the 1-bit method, that is, a so-called 1-bit amplifier has a merit that, unlike the PCM method, it does not require a D/A converter and is able to reproduce the original analog signal by a simple process of just eliminating the digital signal of a high-frequency component with a low-pass filter provided in a final stage.
FIG. 1 is a block diagram schematically showing a configuration of the 1-bit amplifier in the past. In FIG. 1, a ΔΣ modulation portion 52 converts a digital audio 1-bit signal reproduced from a CD 51 based on the ΔΣ modulation so as to obtain a PWM (Pulse Width Modulation) signal. And it supplies the obtained PWM signal to a driver circuit 53, and utilizes it as a control signal for driving a power amplifier 54.
The power amplifier 54 is comprised of a full-bridge switching circuit, and controls ON-state time of the switching elements (MOS transistors Q1 to Q4) so as to amplify an audio signal based on a supplied power-supply voltage Vp and output it. The PWM signal having analog-like width on time base is used as the signal for controlling the switching.
The audio signal amplified by the power amplifier 54 turns to an analog audio signal through low-pass filters (LPF) 55 and 56 comprised of coils L1 and L2 and capacitors C1 and C2 so as to be outputted from a speaker 57. At this time, as shown in FIG. 1, the power amplifier 54 has two MOS transistors Q1 and Q4 and two MOS transistors Q2 and Q3 pair up respectively to be alternately turned on. Thus, a voltage given to the coils of the speaker 57 is allocated to a positive and a negative so as to output the audio signal.
As described above, it is possible, by using the 1-bit amplifier of a configuration as in FIG. 1, to reproduce the original analog signal in a simple process of just eliminating a high-frequency signal with the low-pass filters 55 and 56 without performing D/A conversion on reproduction. In the case of such a configuration, however, there is a problem that a DC offset voltage is generated in a bridge circuit of the power amplifier 54 due to variations in the characteristics of the four MOS transistors Q1 to Q4 so that reproduced sound quality is degraded.
When amplifying the audio signal with the bridge circuit of the power amplifier 54 and outputting it from the speaker 57 under ordinary circumstances, it is necessary to allocate an output voltage to the speaker 57 to the positive and negative centering on null voltage without an offset. However, there is a problem that, if an offset voltage is generated in the bridge circuit, when outputting a loud sound for instance, an output level thereof hits the peak and becomes clipped to distort a waveform of the audio signal, causing an adverse effect on the reproduced sound quality.
The power amplifier 54 of a bridge type is usually designed so that when the pair of MOS transistors Q1 and Q4 is on, the other pair of MOS transistors Q2 and Q3 must be off. However, there is a problem that when switching an ON operation of the pair of MOS transistors Q1 and Q4 and the other pair of MOS transistors Q2 and Q3, there arises a state in which both become on due to the variations in the characteristics (switching speed and so on) of the four MOS transistors Q1 to Q4 and a through-current runs through the MOS transistors Q1 and Q2 or the MOS transistors Q3 and Q4.
The present invention was implemented to solve such problems, and its object is to be able to effectively cancel the offset voltage and through-current generated due to the variations in the characteristics of the switching elements constituting the bridge type power amplifier.